The present invention is related to electrochemical devices, in particular to printable, electrochemical pixel devices based on conducting organic materials and electrochromic materials. The invention also relates to a process for the production of an electrochemical pixel device, and to matrices of electrochemical pixel devices.
Semiconducting and conducting organic materials, both polymers and molecules, have successfully been included in a large range of electronic devices, e g electrochemical devices, for instance as dynamic colorants in smart windows and in polymer batteries. Reversible doping and de-doping involving mobile ions:switches the material between different redox states.
Use has been made of semiconducting polymers for the realisation of field effect transistor (FET) devices. The transistor channel of these devices comprises the semiconducting polymer in question, and their function is based on changes in charge carrier characteristics in the semiconducting polymer, caused by an externally applied electric field. In such transistors, the polymer is used as a traditional semiconductor, in that the electric field merely redistributes charges within the polymer material. One such transistor has been realised, which is adapted for miniaturisation and can be used for the production of integrated circuits consisting entirely of polymer material (PCT publication WO99/10939). A stack of sandwiched layers is described, with either a top-gate or a bottom-gate structure. A transistor device with a similar architecture, also using a polymer as semiconducting material in the channel of the transistor, is described in the European patent application EP1041653.
Another type of transistor device based on organic materials utilises electrochemical redox reactions in the organic material. These devices comprise an electrolyte and a conducting polymer that can be switched between an oxidised and a reduced state. One of these oxidation states then corresponds to low, preferably zero, conductivity in the material, whereas the other oxidation state corresponds to a high conductivity relative to the first state. Electrochemical transistor devices have been used as sensors, e g for detection of oxidant in a solution (see, for review, Baughman and Shacklette, Proceedings of the Sixth Europhysics Industrial Workshop (1990), p 47-61). Furthermore, a transistor of the electrochemical type is reported in Rani et al, J Solid State Electrochem (1998), vol 2, p 99-101. The gate electrode architecture in this prior art transistor is shown in FIG. 1 of this reference.
Electrochromic materials exhibit colour changes or changes in optical density as a result of electrochemical reduction and/or oxidation reaction;. An electrochromic material can either be present as a solid, or exist as molecular, neutral or ionic species in an electrolyte solution. These materials have been used for the creation of electrochromic cells, where the passage of electric charge causes colour changes in the materials. Electrochromic cells are used in electrochromic devices of different kinds, and two principal categories of these devices can be distinguished. The two categories differ from each other mainly in the arrangement of the elements of the electrochromic cell.
The first category of electrochromic devices utilises a sandwich construction, and is used in applications such as automobile windows, building windows, sunglasses, large billboards, mirrors with variable reflectance, sunroofs etc. In this type of electrochromic device, continuous layers of electrochromic material and electrolyte (as well as other layers of e g ion reservoir material) are confined between two electrodes that completely cover the layers of electrochromic material and electrolyte. For the electrochromic device to be of use, at least one of said electrodes has to be transparent to let light through the device. This requirement is met in the prior art through the use of electrode materials such as indium-doped tin oxide (ITO), tin dioxide or fluorine-doped tin dioxide. The electrochromic materials used in these applications vary, but are often based on heavy metal oxides such as WO3 or conducting polymers such as polyaniline or polypyrrole. The conducting, electrochromic polymer poly-(3,4-ethylendioxythiophene) (PEDOT) has attracted much study, and sandwich devices incorporating this polymer have been realised.
The second category of electrochromic devices aim at providing an electrically updateable display for realisation on a flexible support. U.S. Pat. No. 5,754,329 describes such a display, in which the electrodes of the electrochromic device are placed in one and the same plane, contacting a layer of electrochromic material for the generation of local colour effects at the interface between the electrochromic material and the electrodes. U.S. Pat. No. 5,877,888 represents a further development of this device, describing a two-sided display. However, the arrangement of the component layers of the electrochromic device is similar to that of the device of the U.S. Pat No. 5,754,329, considering that the electrodes on either side of the display support contact electrochromic material only, and the generation of electrochromic effects is confined to the area of the electrodes. The electrochromic materials that are used in these devices are described in detail in U.S. Pat. No. 5,812,300.
Active addressing of pixel displays is described e g by Firester A H, in xe2x80x9cActive Matrix Technologyxe2x80x9d, chapter 5 of xe2x80x9cFlat-Panel Display Technologiesxe2x80x9d, eds Tannas, Glenn, Doane et al, 1995 (ISBN 0-8155-1387-9) The prior art in this field may furthermore be represented by U.S. Pat. Nos. 6,157,356, 6,023,259 and 6,072,517. Active matrix driven OLED-displays (organic light emitting displays) have been realised by e g Philips and Cambridge Display Technology (CDT). In parallel to what was noted in connection with transistors above, the transistors used in all of these applications, when employing a polytler material, utilise the polymer material as a traditional semiconductor.
Problems with the pixel matrices in the displays of the prior art mentioned above include the fact that they are difficult and expensive to manufacture. In particular, no electrochemical pixel devices have been disclosed which are truly capable of being mass produced. Furthermore, the practical use of the pixel elements in the prior art devices has been hampered by their comparatively high power consumption. Also, materials used in prior art devices suffer from a lack of environmental friendliness, processability and economic production possibilities. There is therefore a need for new and improved pixel devices for incorporation in matrices that may be used in displays.
One of the objects of the present invention is then to meet this demand, by developing the art of electrochemical pixel devices, and by providing a device with handling, production, disposal and other characteristics superior to those of the prior arts.
Another object of the present invention is to provide an electrochemical pixel devict which can be deposited on a large range of different rigid or flexible substrates by conventional printing methods.
Yet another object of the present invention is to provide an environmentally safe electrochemical pixel device, so that the disposal of the device, along with any support onto which it has been deposited, doesn""t give rise to handling problems, and so that no safety restrictions have to be imposed on the use of the device.
Another object of the present invention is to provide an actively addressed matrix display, offering versatility and ease of design, and enabling a good picture quality.
Still another object of the present invention is to make possible new applications of conpducting organic materials, using several different properties of such materials in combination.
A further object of the invention is to provide processes for the production of such devices, which processes utilise conventional printing methods or other deposition techniques that are well known, relatively unexpensive and easily scaled up.
The aforementioned objects are met by the invention as defined in the independent claims. Specific embodiments of the invention are defined in the dependent claims. In addition, the present invention has other advantages and features apparent from the detailed description below.
Thus, an electrochemical pixel device is provided, which comprises the following components:
(A) an electrochemical transistor device comprising: a source contact; a drain contact; at least one gate electrode; an electrochemically active element arranged between, and in direct electrical contact with, the source and drain contacts, which electrochemically active element comprises a transistor channel and is of a material comprising an organic material having the ability of electrochemically altering its conductivity through change of redox state thereof; and a solidified electrolyte in direct electrical contact with the electrochemically active element and said at least one gate electrode and interposed between them in such a way that electron flow between the electrochemically active element and said gate electrode(s) is prevented; whereby flow of electrons between source contact and drain contact is controllable by means of a voltage applied to said gate electrodes), and
(B) an electrochromic device comprising: at least one electrochromic element comprising (i) at least one material that is electrically conducting in at least one oxidation state and (ii) at least one electrochromic material, wherein said materials (i) and (ii) can be the same or different; at least one layer of a solidified electrolyte which is in direct electrical contact with said electrochromic element; and at least two electrodes adapted for application of a voltage therebetween; each of said electrodes being in direct electrical contact with a component selected from said electrolyte layer(s) and said electrochromic element;
in which pixel device one of said source and drain contacts of the electrochemical transistor device A is in electrical contact with one of the electrodes of the electrochromic device B.
Thus, the invention is based on the finding that a so called xe2x80x9csmart pixelxe2x80x9d, comprising a combination of a transistor and a colour cell, may be assembled by the novel components as described above. Depending on the electrochemical properties of the precise materials used in the electrochemical transistor device A and the electrochromic device B, the electrochromic device P may be connected to either of the source aid drain contacts of the electrochemical transistor device A. The electrochemical pixel device is advantageous in that a voltage applied to the electrochromic display B for colouring or decolouring the electrochromic element may be modulated by controlling the gate voltage of the electrochemical transistor device A. This beneficial property of the electrochemical pixel device of the invention makes it suitable for construction of matrix assemblies of such pixels. The electrochemical pixel devices of the invention may thus be used to form a matrix that may be actively addressed, in a manner known per se in the art.
Electrochemical Transistor Device A
The architecture of the electrochemical transistor device A in the electrochemical pixel device according to the invention is advantageous in that it makes possible the realisation of a layered transistor device with only a few layers, having for example one patterned layer of material comprising a conducting organic material, which layer comprises source and drain contacts and gate electrode(s), as well as the electrochemically active element. The source and drain contacts and the electrochemically active element are then preferably formed by one continuous piece of said material. The source and drain contacts could alternatively be formed from another electrically conducting material in direct electrical contact with the electrochemically active element. The gate electrode(s) may also be of another electrically conducting material. To provide for the necessary electrochemical reactions, whereby the conductivity in the active element is changed, a solidified electrolyte is arranged so that it is in direct electrical contact with both the active element and the gate electrode(s).
In a preferred embodiment, the source and drain contacts and gate electrode(s), as well as the active element, are all arranged to lie in a common plane, further simplifying production of the device by ordinary printing methods. Thus, the electrochemical device according to this embodiment of the invention uses a lateral device architecture. A layer of solidified electrolyte can advantageously be deposited so that it covers, at least partly, the gate electrodes) as well as covering the electrochemically active element. This layer of solidified electrolyte may be continuous or interrupted, depending partly on which of two main types of transistor architectures is to be realised (see below).
The electrochemical transistor device A in the electrochemical pixel device according to the invention allows for control of electron flow between source and drain contacts, and thus for control of the current through the electrochromic element of the electrochromic device B. The conductivity of the transistor channel of the electrochemically active element can be modified, through altering of the redox state of the organic material therein. This is achieved by a application of a voltage to the gate electrode(s), which generates an electric field in the electrolyte. In the contact area between electrolyte and electrochemically active element, electrochemical redox reactions take place, which change the conductivity of the organic material. Either the organic material in the transistor channel is modified from a conducting state to a non-conducting state as a result of said redox reactions, or it is modified from a non-conducting to a conducting state.
As is readily appreciated by the skilled person, and in analogy to conventional field effect transistors, the electrochemical transistor device A in the electrochemical pixel device of the invention may readily be made to function as a diode device through short-circuiting of the gate electrode and source contact, or of the gate electrode and drain contact. Any configuration of the electrochemical transistor device A may be used as a diode in this fashion.
Depending on the precise patterning of the conducting organic material and the electrolyte, the electrochemical transistor device A in the electrochemical pixel device of the invention can either be of a bi-stable or a dynamic type. In the bi-stable transistor embodiment, a voltage applied to the gate electrodes leads to a change in conductivity in the transistor channel that is maintained when the external circuit is broken, i e when the applied voltage is removed. The electrochemical reactions induced by the applied voltage can not be reversed, since the electrochemically active element and the gate electrode(s) are not in direct electrical contact with each other, but separated by electrolyte. In this embodiment, the transistor channel can be switched between non-conducting and conducting states using only small, transient gate voltages. The bi-stable transistor can be kept in an induced redox state for days, and, in the most preferred, ideal case, indefinitely.
Thus, the bi-stable embodiment of the electrochemical transistor device A offers a memory function, in that it is possible to switch the transistor on or off using only a short voltage pulse applied to the gate electrode. The transistor stays in the conducting or non-conducting redox state even after the applied voltage has been removed. A further advantage with such bi-stable transistors is that close to zero-power operation is made possible, since the short voltage pulses applied to the gate need not be larger than a fraction of the gate voltages needed for operation of a corresponding dynamic device.
In the dynamic embodiment of the electrochemical transistor device A, the change in the redox state of the material is reversed spontaneously upon withdrawal of the gate voltage. This reversal is obtained through the provision of a redox sink volume adjacent to the transistor channel in the electrochemically active element. Also, a second gate electrode is provided, and arranged so that the two gate electrodes are positioned on either side of the electrochemically active element, one closer to the transistor channel, and the other closer to the redox sink volume. Both gate electrodes are separated from the electrochemically active element by electrolyte. Application of a voltage between the two gate electrodes results in the electrochemically active element being polarised, whereby redox reactions take place in which the organic material in the transistor channel is reduced while the organic material in the redox sink volume is oxidised, or vice versa. Since the transistor channel and the redox sink volume are in direct electrical contact with each other, withdrawal of gate voltage leads to a spontaneous reversal of the redox reactions, so that the initial conductivity of the transistor channel is re-established. It is to be stressed that in contrast to electrochemical transistors of the prior art, dynamic transistors comprised in this embodiment of the present invention revert spontaneously to the initial conductivity state without the need for a reversing bias.
Electrochromic Device B
The electrochromic device B in the electrochemical pixel device according to the invention comprises an electrochromic element in direct electrical contact with an electrolyte. At least two electrodes of a voltage supply are each in direct electrical contact with either of the electrolyte or electrochromic element, in order to enable supply of charge carriers for colouring, or decolouring, electrochemical reactions. The electrodes may be formed from the same material as the electrochromic element. The contact of electrodes to the electrochromic element, whether directly or via electrolyte, may be made through a lateral or a vertical arrangement, depending on the design of the electrochemical pixel device according to the invention.
In one embodiment, an electrochromic device B of the electrochemical pixel device is provided, in which the electrodes are arranged side by side, in a plane. The electrodes then form an electrode layer, which can be deposited on a support in a conventional manner, and patterned in any desirable fashion. When this arrangement of electrodes is used, the connections formed with the electrolyte or electrochromic element are preferably made with only one layer of said electrolyte or electrochromic element. Alternatively, one of the electrodes is positioned laterally in relation to the electrochromic element, whereas a second electrode is positioned underneath, separated from the electrochromic element by electrolyte. Such a vertical design may be advantageous in the construction of electrochemical pixel devices, since, in this arrangement, several of the components of the electrochromic device, excepting of course the electrochromic element, may be hidden from the viewer. This arrangement of electrochromic element on top of the electrolyte and second electrode is particularly advantageous in the context of a matrix of a plurality of electrochemical pixel devices, which matrix forms a pixelated display. The fact that a maximum portion of the visible face of such a display consists of electrochromic elements gives a maximal picture quality and clarity. Also, the construction of a matrix of electrochemical pixel devices is made simpler with this vertical arrangement of the electrodes of the electrochromic device B, since fewer crossings of conductors with each other need to be designed. This aspect of the design of the electrochromic device B of the electrochemical pixel device of the invention will be discussed in greater detail below.
In some embodiments of the invention, the electrolyte in the electrochromic device B is in the form of a continuous layer, giving rise to a dynamic device in which application of voltage results in a colour change that is reversed upon removing the voltage. In other embodiments of the present invention, an electrochromic device is provided in which the electrolyte is patterned between electrodes. The conduction of ions in this device is then interrupted, so that the application of voltage to the electrochemical cell of the device results in reduction and oxidation reactions that are not reversed upon removing the voltage. Thus, bi-stable switching between states is made possible by these accumulator-like properties of such embodiments of the device.
In embodiments of the invention, an electrochromic device is provided, which comprises at least one further electrochromic material to complement said electrochromic material in the electrochromic element. This makes it possible to realise devices with more than one colour, with for example one colour-generating oxidation reaction and one colour-generating reduction reaction taking place simultaneously at different locations in the device. As a further example, redox reactions giving rise to different colours at the same location, but at different applied voltages, can be designed. This further electrochromic material can be provided within the solidified electrolyte or within the electrochromic element, which then for example comprises an electrochromic redox pair.
Embodiments of the electrochronic device B in the electrochemical pixel device of the invention may also comprise a redox active material which does not in itself give rise to electrochromic effects. Such a material may fulfil any or both of the following two roles: (i) In some arrangements of the electrochrcmic device B, the electrochromic material of the entire volume of the electrochromic element can not be completely oxidised or reduced in the absence of a complementary redox reaction; rather, only part of the material will be oxidised or reduced, respectively. Thus, the addition of a further redox active material makes it possible to fully oxidise or reduce the electrochromic material; (ii) The electrocromic material may be sensitive to over-oxidation, occurring at too high an applied voltage, and destroying the electrochromic material rendering it useless. A further redox active material comprised in the device may serve the function of protecting the electrocromic material from such over-oxidation, through restricting the electric polarisation in the electrochromic element to a value below a threshold value. At this threshold value, the protective, further redox active material will instead be oxidised, protecting the electrochromic material from a polarisation that would otherwise destroy it. As is readily appreciated by the skilled man in the light of what is discussed above, a suitably chosen redox active material, exhibiting electrochromic effects, could serve the function of providing a complementary, colour-generating reaction, at the same time as it provides either or both of the beneficial effects of protection against over-oxidation and enabling of complete reduction/oxidation of the first electrochromic material.
In some embodiments of the electrochromic device B in the electrochemical pixel device of the invention, dynamic or variable colouring effects in the electrochromic device may be generated through use of a combination of different solidified electrolytes, having different ionic conductivities. Parts of an electrochromic element, or some of a plurality of electrochromic elements, may then be in direct electrical contact with such different electrolytes. Electrochromic areas that are in contact with an electrolyte having higher ionic conductivity will colour/decolour faster than electrochromic areas that are in contact with an electrolyte having a lesser ionic conductivity, which makes possible different combinations of image elements with different colouring and decolouring speeds.
As described above, in preferred embodiments of the invention, the source and drain contacts, the electrochemically active element and the gate electrodes of the electrochemical transistor device A are all formed from the same material. Suitably, and provided that the material has electrochromic properties, this material is also used for the formation of the electrochromic element in the electrochromic device B, and, even more preferably, also for the formation of the electrochromic device electrodes. Suitably, in view of the various desirable properties of said material, the common material chosen for the different components is an electrochromic polymer which is conducting in at least one oxidation state. Suitable such polymers are listed in the xe2x80x9cMaterialsxe2x80x9d section below.
For the successful operation of the electrochemical pixel device, both the electrochemical transistor device A and the electrochromic device B comprise a solidified electrolyte. In A, the electrolyte makes possible the electrochemical reactions that chance the conductivity in the transistor channel, whereas in B, it enables the electrochemical reactions resulting in a colour change in the electrochromic element. The solidified electrolytes in A and B may be the same or different, and is as defined in the xe2x80x9cMaterialsxe2x80x9d section below. For reasons of processability and ease of design, the solidified electrolytes in A and B are preferably the same.
Another aspect of the invention provides a matrix of electrochemical pixel devices as described above. The beneficial process characteristics of the electrochemical pixel device makes it suitable for incorporation in a matrix comprising a plurality of similar or identical pixel devices, in order to form an actively addressed pixel matrix. As is known to the person skilled in the art of displays incorporating actively addressed matrices, such a matrix of pixels exploits the possibility provided by a transistor coupled to a pixel to control the current flows to the colour elements of the pixels. If a matrix according to this aspect of the invention consists of, for example, three rows and three columns of the novel electrochemical pixel devices described herein, there will be nine electrochromic devices b whose colour may be changed individually. There will also be nine electrochemical transistor devices A to control the current. One row or column of the matrix may be updated at a time. In other words, e g all the transistors in the two lowest rows are set to the xe2x80x9coffxe2x80x9d state, and are thus in a non-conducting state. The transistors in the upper row are set to the conducting or xe2x80x9conxe2x80x9d state, so that the colour of all electrochromic devices in this row may be individually changed. After completion of the process for the displays in the first row, the transistors in the first and third row will be set to their xe2x80x9coffxe2x80x9d states, whereas the transistors in the second row are set to their xe2x80x9conxe2x80x9d state in order that the displays in the second row may now be individually updated. By setting the transistors in a row to the xe2x80x9coffxe2x80x9d state after updating of this row, the charges loaded into the electrochromic elements in question will be trapped there, which in turn means that a certain electrochromic element can stay in its colour changed state for some time. In summary, the transistors make control of the current applied to the electrochromic elements possible, and thereby also the selection of individual display cells to be switched. The skilled person may create a matrix of electrochemical pixel devices according to the invention, having any number of pixels without undue experimentation, using the teachings herein. Specific embodiments showing alternative matrix designs and variants of the electrochemical pixel device to be used in such matrices are shown in the detailed description below. Especially, the electrochemical transistor devices A in the pixels forming part of such a matrix may have one or two electrodes for supply of gate voltage to the transistor channel. Different, characteristics are obtained in the different cases, as explored in further detail below. Briefly, a design having two gate electrodes generally provides for better control of the currents in the matrix system. However, a design of only one gate electrode prevents cross-talk between the second gate electrodes of all the pixels in a row, which may occur as a disadvantage in the two-gate design. The one-gate design also makes for fewer crossings of conductors. The problem of cross-talk between gate electrodes in a row may also be addressed by the provision of resistances, which prevent the current from a certain pixel from spreading to other pixels in the same row.
The invention in a further aspect also extends to a pixelated display which comprises such a matrix of electrochemical pixel devices.
The electrochemical pixel device according to the invention is advantageous in that it can be easily realised on a support, such as polymer film or paper. Thus, the different components can be deposited on the support by means of conventional printing techniques such as screen printing, offset printing, ink-jet printing and flexographic printing, or coating techniques such as knife coating, doctor blade coating, extrusion coating and curtain coating, such as described in xe2x80x9cModern Coating and Drying Technologyxe2x80x9d (1992), eds E D Cohen and E B Gutoff, VCH Publishers Inc, New York, N.Y., USA. In those embodiments of the invention that utilise a polymer as the organic material in the electrochemical transistor device A or as the electrochromic material in the electrochromic device a (see below for materials specifications), this material can also be deposited through in situ polymerisation by methods such as electropolymerisation, UV-polymerisation, thermal polymerisation and chemical polymerisation. As an alternative to these additive techniques for patterning of the components, it is also possible to use subtractive techniques, such as local destruction of material through chemical or gas etching, by mechanical means such as scratching scoring, scraping or milling, or by any other subtractive methods known in the art. Yet another aspect of the invention provides such processes for the manufacture of an electrochemical pixel device from the materials specified herein.
According to a preferred embodiment of the invention, the electrochemical pixel device is encapsulated, in part or entirely, for protection of the device. The encapsulation retains any solvent needed for e g the solidified electrolyte to function, and also keeps oxygen from disturbing the electrochemical reactions in the device. Encapsulation can be achieved through liquid phase processes. Thus, a liquid phase polymer or organic monomer can be deposited on the device using methods such as spray-coating, dip-coating or any of the conventional printing techniques listed above. After deposition, the encapsulant can be hardened for example by ultraviolet or infrared irradiation, by solvent evaporation, by cooling or through the use of a two-component system, such as an epoxy glue, where the components are mixed together directly prior to deposition. Alternatively, the encapsulation is achieved through lamination of a solid film onto the electrochemical pixel device. In preferred embodiments of the invention, in which the components of the electrochemical pixel device are arranged on a support, this support can function as the bottom encapsulant. In this case encapsulation is made more convenient in that only the top of the sheet needs to be covered with liquid phase encapsulant or laminated with solid film.
Further objects and purposes of the present invention will be clear from the following drawings and detailed description of specific embodiments thereof. These specifications and drawings are intended as illustrations of the invention as claimed, and are not to be seen as limiting in any way.